Voice Over Ethernet 99 www.newnespress.com too full, but the fact that it stops dropping when the queue empties gives pressure when the queues are filling and permissiveness when there is plenty of room. On top of RED is a concept called weighted random early detection (WRED). WRED uses weights, based on the classifications we have seen already, to alter the drop probabilities. Using the classifications for voice allows administrators to avoid having WRED kick in for voice, which is inelastic and will not respond to being dropped, if the administrator has no ability to place voice in a separate queue or route. For data, more critical data connections, such as TCP-based SIP needed in calls, can be given a higher probability by avoiding a higher drop probability, while allowing normal data to be slowed down. The problem with RED is the problem with policing. Packets that may have been needed to prevent the queue from going idle even though there are resources for them, causing lost work and wasted resources. 4.2.3.9 Explicit Congestion Notification Instead of using RED, routers have the option of marking the packets, rather than dropping them. TCP endpoints that know to read for the congestion-marked packets will consider it as if the packet had, somehow, been lost, and will back off or slow down, but without causing the packet’s data to disappear. This increases the performance of the network and improves efficiency, though, needless to say, it does nothing if the endpoints are not aware of the congestion notification scheme. On TCP, explicit congestion notification (ECN) works by the TCP endpoints negotiating that they support this protocol. Both sides need to support it, because the only way the sender can know if an intervening router has marked a packet is for the receiver to echo that fact back to the sender over TCP itself. Once a flow is established, the sender sets the ECN bit, bit 6, in the DSCP usage of the TOS field in IP (Section 4.2.2, Table 4.14). This lets routers know that the packet supports ECN. When a router uses RED to decide that the packet should be dropped early, but notices that the packet is marked for ECN support and the router supports ECN itself, it will not drop the packet. Instead, it will set the seventh bit in the ECN header, the CE or Congestion Experienced bit, marking that the packet should be handled as if it were to have been dropped. The TCP receiver notices that the packet has been marked, and so needs to echo this fact back in the acknowledgment. The receiver sets the ECE, or ECN-echo bit in the TCP flags field in the acknowledgement. (Section 4.1.1.4, Table 4.10). The sender gets the acknowledgement, and uses this flag to cut its congestion window in half, as if the original packet were lost. 101 CHAPTER 5 Introduction to Wi-Fi 5.0 Introduction This chapter provides an introduction into wireless local area networking based on Wi-Fi, also known by its more formal standard name of IEEE 802.11. The goal of the chapter is to provide a solid background on Wi-Fi technology, looking at what needs to be done to ensure that wireless local area networks operate well as both a data network and a crucial leg of voice mobility solutions. This chapter is aimed for readers with all degrees of familiarity with wireless networking. Although not a reference on all things Wi-Fi, the chapter starts with the basics of Wi-Fi before diving into what makes voice unique over this particular type of network. 5.1 The Advantages of Wi-Fi Until now, we’ve looked at why voice is interesting and what makes it work over a network, but we haven’t yet examined the technologies that truly make voice mobile. The advantage of mobile voice, when working properly, is that the elements of the underlying network fade away, and user sees only a familiar phone, in a mobile package. Of course, this requires cutting the cord, allowing users to make or receive calls from anywhere. So that we can understand how and why an unwired network is able to make the elements of the network disappear to the user, so to speak, we need to dive deeper and understand what the unwired network is made of. Wi-Fi, the wireless local area networking technology based on the work from the standards branch of the Institute of Electrical and Electronics Engineers, uses the IEEE 802.11 standard to allow portable mobile devices to connect to each other over the air, transmitting IP-based data as if they were connected directly with a cable. But being wireless alone does not explain why Wi-Fi has become the primary wireless technology for both consumer- and enterprise-owned networks. Wi-Fi technology has a number of advantages that make it the obvious choice for wireless data, and for many circumstances, for mobile voice as well. ©2010 Elsevier Inc. All rights reserved. doi:10.1016/B978-1-85617-508-1.00001-3. 102 Chapter 5 www.newnespress.com 5.1.1 Unlicensed Spectrum Generally, the ability to transmit radio signals over the air is tightly regulated. Government bodies, such as the U.S. Federal Communications Commission (FCC), determine what technologies can be used to transmit over the air and who is allowed to operate those technologies (see Figure 5.1). They do this latter part by issuing licenses, usually for money, to organizations interested in transmitting wirelessly. These licenses, which are often hard to obtain, are required in part to prevent multiple network operators from interfering with each other. The advantage of Wi-Fi, over other wireless technologies such as WiMAX (which we will cover in Chapter 7), is that no licenses are needed to set up and operate a Wi-Fi network. All that it takes to become a network operator is to buy the equipment and plug it in. What is the Difference between Wi-Fi and IEEE 802.11? Almost everywhere, the term Wi-Fi is now used to refer to the networking technology based on the IEEE 802.11 standard. There are subtle differences, however, between the two terms. The term Wi-Fi is a trademark of the Wi-Fi Alliance, a nonprofit industry organization made up of nearly all of the equipment providers manufacturing IEEE 802.11–based devices: chipset vendors, consumer and enterprise access point vendors, computer manufacturers, and so on. The Wi-Fi Alliance exists for two reasons: to promote the use of Wi-Fi certified technology throughout the industry and within the press, and to ensure that wireless devices based on 802.11 work together. The term Wi-Fi, and the accompanying logo, can be used only for products that have passed the Wi-Fi Alliance’s certification programs. We’ll discuss the Wi-Fi Alliance more later, and where the Wi-Fi Alliance’s certification programs diverge from the IEEE 802.11 standard. In the meantime, remember that 802.11 and Wi-Fi mean almost the same thing. Example of the Wi-Fi Alliance Certification Logo. Note: the logo is the trademark of the Wi-Fi Alliance and is shown here for example purposes only. U . S . D E P A R T M E N T O F C O M M E R C E N A T I O N A L T E L E C O M M U N I C A T I O N S & I N F O R M A T I O N A D M I N I S T R A T I O N MOBILE (AERONAUTICAL TELEMETERING) S) 5.68 5.73 5.90 5.95 6.2 6.525 6.685 6.765 7.0 7.1 7.3 7.35 8.1 8.195 8.815 8.965 9.040 9.4 9.5 9.9 9.995 10.003 10.005 10.1 10.15 11.175 11.275 11.4 11.6 11.65 12.05 12.10 12.23 13.2 13.26 13.36 13.41 13.57 13.6 13.8 13.87 14.0 14.25 14.35 14.990 15.005 15.010 15.10 15.6 15.8 16.36 17.41 17.48 17.55 17.9 17.97 18.03 18.068 18.168 18.78 18.9 19.02 19.68 19.80 19.990 19.995 20.005 20.010 21.0 21.45 21.85 21.924 22.0 22.855 23.0 23.2 23.35 24.89 24.99 25.005 25.01 25.07 25.21 25.33 25.55 25.67 26.1 26.175 26.48 26.95 26.96 27.23 27.41 27.54 28.0 29.7 29.8 29.89 29.91 30.0 UNITED STATES THE RADIO SPECTRUM NON-GOVERNMENT EXCLUSIVE GOVERNMENT/ NON-GOVERNMENT SHARED GOVERNMENT EXCLUSIVE RADIO SERVICES COLOR LEGEND ACTIVITY CODE NOT ALLOCATED RADIONAVIGATION FIXED MARITIME MOBILE FIXED MARITIME MOBILE FIXED MARITIME MOBILE Radiolocation RADIONAVIGATION FIXED MARITIME MOBILE Radiolocation FIXED MARITIME MOBILE FIXED MARITIME MOBILE AERONAUTICAL RADIONAVIGATION AERONAUTICAL RADIONAVIGATION Aeronautical Mobile Maritime Radionavigation (Radio Beacons) MARITIME RADIONAVIGATION (RADIO BEACONS) Aeronautical Radionavigation (Radio Beacons) 3 9 14 19.95 20.05 30 30 59 61 70 90 110 130 160 190 200 275 285 300 3 kHz 300 kHz 300 kHz 3 MHz 3 MHz 30 MHz 30 MHz 300 MHz 3 GHz 300 GHz 300 MHz 3 GHz 30 GHz Aeronautical Radionavigation (Radio Beacons) MARITIME RADIONAVIGATION (RADIO BEACONS) Aeronautical Mobile Maritime Radionavigation (Radio Beacons) AERONAUTICAL RADIONAVIGATION (RADIO BEACONS) AERONAUTICAL RADIONAVIGATION (RADIO BEACONS) Aeronautical Mobile Aeronautical Mobile RADIONAVIGATION AERONAUTICAL RADIONAVIGATION MARITIME MOBILE Aeronautical Radionavigation MOBILE (DISTRESS AND CALLING) MARITIME MOBILE MARITIME MOBILE (SHIPS ONLY) MOBILE AERONAUTICAL RADIONAVIGATION (RADIO BEACONS) AERONAUTICAL RADIONAVIGATION (RADIO BEACONS) BROADCASTING (AM RADIO) MARITIME MOBILE (TELEPHONY) MARITIME MOBILE (TELEPHONY) MOBILE (DISTRESS AND CALLING) MARITIME MOBILE LAND MOBILE MOBILE FIXED STANDARD FREQ. AND TIME SIGNAL (2500kHz) STANDARD FREQ. AND TIME SIGNAL Space Research MARITIME MOBILE LAND MOBILE MOBILE FIXED AERONAUTICAL MOBILE (R) STANDARD FREQ. AERONAUTICAL MOBILE (R) AERONAUTICAL MOBILE (OR) AERONAUTICAL MOBILE (R) FIXED MOBILE** Radio- location FIXED MOBILE* AMATEUR FIXED FIXED FIXED FIXED FIXED MARITIME MOBILE MOBILE* MOBILE* MOBILE STANDARD FREQ. AND TIME SIGNAL (5000 KHZ) AERONAUTICAL MOBILE (R) AERONAUTICAL MOBILE (OR) STANDARD FREQ. Space Research MOBILE** AERONAUTICAL MOBILE (R) AERONAUTICAL MOBILE (OR) FI XED MOBILE* BROADCASTING MARITIME MOBILE AERONAUTICAL MOBILE (R) AERONAUTICAL MOBILE (OR) FIXED Mobile AMATEUR SATELLITE AMATEUR AMATEUR FIXED Mobile MARITIME MOBILE MARITIME MOBILE AERONAUTICAL MOBILE (R) AERONAUTICAL MOBILE (OR) FIXED BROADCASTING FIXED STANDARD FREQ. AND TIME SIGNAL (10,000 kHz) STANDARD FREQ. Space Research AERONAUTICAL MOBILE (R) AMATEUR FIXED Mobile* AERONAUTICAL MOBILE (R) AERONAUTICAL MOBILE (OR) FIXED FIXED BROADCASTING MARITIME MOB ILE AERONAUTICAL MOBILE (R) AERONAUTICAL MOBILE (OR) RADIO ASTRONOMY Mobile* AMATEUR BROADCASTING AMATEUR AMATEUR SATELLITE Mobile* FIXED BROADCASTING STANDARD FREQ. AND TIME SIGNAL (15,000 kHz) STANDARD FREQ. Space Research FIXED AERONAUTICAL MOBILE (OR) MARITIME MOB ILE AERONAUTICAL MOBILE (OR) AERONAUTICAL MOBILE (R) FIXED FIXED BROADCASTING STANDARD FREQ. Space Research FIXED MARITIME MOBILE Mobile FIXED AMATEUR AMATEUR SATELLITE BROADCASTING FIXED AERONAUTICAL MOBILE (R) MARITIME MOBILE FIXED FIXED FIXED Mobile* MOBILE** FIXED STANDARD FREQ. AND TIME SIGNAL (25,000 kHz) STANDARD FREQ. Space Research LAND MOBILE MARITIME MOBILE LAND MOBILE MOBILE** RADIO ASTRONOMY BROADCASTING MARITIME MOBILE LAND MOBILE FIXED MOBILE** FIXED MOBILE** MOBILE FIXED FIXED FIXED FIXED FIXED LAND MOBILE MOBILE** AMATEUR AMATEUR SATELLITE MOBILE LAND MOBILE MOBILE MOBILE FIXED FIXED MOBILE MOBILE FIXED FIXED LAND MOB ILE LAND MOB ILE LAND MOB ILE LAND MOBILE Radio Astronomy RADIO ASTRONOMY LAND MOBILE FIXED FIXED MOBILE MOBILE MOBILE LAND MOBILE FIXED LAND MOB ILE FIXED FIXED MOBILE MOBILE LAND MOBILE AMATEUR BROADCASTING (TV CHANNELS 2-4) FIXED MOBILE FIXED MOBILE FIXED MOBILE FIXED MOBILE AERONAUTICAL RADIONAVIGATION BROADCASTING (TV CHANNELS 5-6) BROADCASTING (FM RADIO) AERONAUTICAL RADIONAVIGATION AERONAUTICAL MOBILE (R) AERONAUTICAL MOBILE AERONAUTICAL MOBILE AERONAUTICAL MOBILE (R) AERONAUTICAL MOBILE (R) AERONAUTICAL MOBILE (R) MOB ILEFIXED AMATEUR BROADCASTING (TV CHANNELS 7-13) MOBILE FIXED MOBILE FIXED MOBILE SATELLITE FIXED MOBILE SATELLITE MOBILE FIXED MOBILE SATELLITE MOBILE FIXED MOBILE AERONAUTICAL RADIONAVIGATION STD. FREQ. & TIME SIGNAL SAT. (400.1 MHz) MET. SAT. (S-E) SPACE RES. (S-E) Earth Expl. Satellite (E-S) MOBILE SATELLITE (E-S) FIXED MOBILE RADIO ASTRONOMY RADIOLOCATION Amateur LAND MOBILE Meteorological Satellite (S-E) LAND MOBILE BROADCASTING (TV CHANNELS 14 - 20) BROADCASTING (TV CHANNELS 21-36) TV BROADCASTING RADIO ASTRONOMY RADIOLOCATION FIXED Amateur AERONAUTICAL RADIONAVIGATION MOBILE**FIXED AERONAUTICAL RADIONAVIGATION Radiolocation Radiolocation MARITIME RADIONAVIGATION MARITIME RADIONAVIGATION Radiolocation Radiolocation Radiolocation RADIO- LOCATION RADIO- LOCATION Amateur AERONAUTICAL RADIONAVIGATION (Ground) RADIO- LOCATION Radio- location AERO. RADIO- NAV.(Ground) FIXED SAT. (S-E) RADIO- LOCATION Radio- location FIXED FIXED SATELLITE (S-E) FIXED AERONAUTICAL RADIONAVIGATION MOBILE FIXED MOBILE RADIO ASTRONOMY Space Research (Passive) AERONAUTICAL RADIONAVIGATION RADIO- LOCATIO N Radio- location RADIONAVIGATION Radiolocation RADIOLOCATION Radiolocation Radiolocation Radiolocation RADIOLOCATION RADIO- LOCATION MARITIME RADIONAVIGATION MARITIME RADIONAVIGATION METEOROLOGICAL AIDS Amateur Amateur FIXED FIXED SATELLITE (E-S) MOBILE FIXED SATELLITE (E-S) F IXED SATELLITE (E-S) MO BI LE FIXED FIXED FIXED FIXED MOBILE FIXED SPACE RESEARCH (E-S) FIXED Fixed MOBILE SATELLITE (S-E) FIXED SATELLITE (S-E) FIXED SATELLITE (S-E) FIXED SATELLITE (S-E) FIXED SATELLITE (S-E) FIXED SATELLITE (E-S) FIXED SATELLITE (E-S) FIXED SATELLITE (E-S) FIXED SATELLITE (E-S) FIXED FIXED FIXED FIXED FIXED FIXED FIXED MET. SATELLITE (S-E) Mobile Satellite (S-E) Mobile Satellite (S-E) Mobile Satellite (E-S) (no airborne) Mobile Satellite (E-S)(no airborne) Mobile Satellite (S-E) Mobile Satellite (E-S) MOBILE SATELLITE (E-S) EARTH EXPL. SATELLITE(S -E) EARTH EXPL. SAT. (S-E) EARTH EXPL. SATELLITE (S-E) MET. SATELLITE (E-S) FIXED FIXED SPACE RESEARCH (S-E) (deep space only) SPACE RESEARCH (S-E) AERONAUTICAL RADIONAVIGATION RADIOLOCATION Radiolocation Radiolocation Radiolocation Radiolocation MARITIME RADIONAVIGATION Meteorological Aids RADIONAVIGATION RADIOLOCATION Radiolocation RADIO- LOCATION Radiolocation Radiolocation Amateur Amateur Amateur Satellite RADIOLOCATION FIXED FIXED FIXED FI XE D FIXED SATELLITE (S-E) FIXED SATELLITE (S-E) Mobile ** SPACE RESEARCH (Passive) EARTH EXPL. SAT. (Passive) RADIO ASTRONOMY SPACE RESEARCH (Passive) EARTH EXPL. SATELLITE (Passive) RADIO ASTRONOMY BROADCASTING SATELLITE AERONAUTICAL RADIONAV. Space Research (E-S) Space Research Land Mobile Satellite (E-S) Radio- location RADIO- LOCATION RADIO NAVIGATION FI XE D SATE LLITE (E-S ) Lan d M obile Sat ellite (E -S) Land Mobile Satellite (E-S) Fixed Mobile FIXED SAT. (E-S) Fixed MobileFIXED Mobile FIXED MOBILE Space Research Space Research Space Research SPACE RESEARCH (Passive) RADIO ASTRONOMY EARTH EXPL. SAT. (Passive) Radiolocation RADIOLOCATION Radiolocation FX SAT (E- S) FIXE D SATE LLITE ( E-S) FI XE D FIXED FI XE D MO BI LE EARTH EXPL. SAT. (Passive) MO BI LE Earth Expl. Satellite (Active) Standard Frequency and Time Signal Satellite (E-S) Earth Exploration Satellite (S-S) MOBILE FIXED MO BI LE FI XE D Earth Exploration Satellite (S-S) FI XE D MO BI LE FI XE D SAT ( E-S) FIXED SATELLITE (E-S) MOBILE SATELLITE (E-S) FIXED SATELLITE (E-S) MOBILE SATELLITE (E-S) Standard Frequency and Time Signal Satellite (S-E) Stand. Frequency and Time Signal Satellite (S-E) FIXED MOBILE RADIO ASTRONOMY SPACE RESEARCH (Passive) EARTH EXPLORATION SAT. (Passive) RADIONAVIGATION RADIONAVIGATION INTER-SATELLITE RADIONAVIGATION RADIOLOCATION Radiolocation SPACE R E. .( Pas siv e) EARTH EXPL. SAT. (Passive) FI XE D MO BI LE FI XE D MO BI LE FI XE D MOBILE Mo bil e Fi xe d FIXED SATELLITE (S-E) BR OA D- CA ST IN G B CS T SA T. FIXED MOBILE F X SAT (E- S) MO BI LE FI XE D EAR TH EXPL ORATIO N SATE LLIT E FI XED SATELLITE (E-S) MOBILE SATELLITE (E-S) MO BI LE FI XE D SP ACE RE SE AR CH (P ass ive ) EARTH EXPLORATION SATELLITE (Passive) EARTH EXPLORATION SAT. (Passive) SPACE RESEARCH (Passive) INTER- SATELLITE RADIO- LOCATION SPACE RESEARCH FI XE D MO BI LE FI XE D MO BI LE SAT EL LIT E (E -S) MOBILE SATELLITE RADIO NAVIGATION RADIO- NAVIGATION SATELLITE EARTH EXPLORATION SATELLITE FI XE D SAT EL LIT E (E -S) MOBILE FIXED FIXED SATELLITE (E-S) AMATEUR AMATEUR SATELLITE AMATEUR AMATEUR SATELLITE Amateur Satellite Amateur RADIO- LOCATION MOBILE FIXED MOBILE SATELLITE (S-E) FIXED SATELLITE (S-E) MOBILEFIXED BROAD- CASTING SATELLITE BROAD- CASTING SPACE RESEARCH (Passive) RADIO ASTRONOMY EARTH EXPLORATION SATELLITE (Passive) MOBILE FI XE D MOBILEFIXED RADIO- LOCATION FIXED SATELLITE (E-S) MOBILE SATELLITE RADIO- NAVIGATION SATELLITE RADIO- NAVIGATION Radio- location EARTH EXPL. SATELLITE (Passive) SPACE RESEARCH (Passive) FI XE D FI XE D SAT EL LIT E (S -E) SPACE RESEARCH (Passive) RADIO ASTRONOMY EARTH EXPLORATION SATELLITE (Passive) FIXED MOBILE MO BI LE INTER- SATELLITE RADIO- LOCATION INTER- SATELLITE Radio- location MOBILE MOBILE SATELLITE RADIO- NAVIGATION RADIO- NAVIGATION SATELLITE AMATEUR AMATEUR SATELLITE Amateur Amateur Satellite RADIO- LOCATION MOBILE FIXED FIXED SATELLITE (S-E) MOBILE FIXED FIXED SATELLITE (S-E) EARTH EXPLORATION SATELLITE (Passive) SPACE RES. (Passive) SPACE RES. (Passive) RADIO ASTRONOMY FIXED SATELLITE (S-E) FIXED MOBILE FIXED MOBILE FIXED MOBILE FIXED MOBILE FIXED MOBILE FIXED SPACE RESEARCH (Passive) RADIO ASTRONOMY EARTH EXPLORATION SATELLITE (Passive) EARTH EXPLORATION SAT. (Passive) SPACE RESEARCH (Passive) INTER- SATELLITE INTER- SATELLITE INTER- SATELLITE INTER- SATELLITE MOBILE MOBILE MOBILE MOBILE SATELLITE RADIO- NAVIGATION RADIO- NAVIGATION SATELLITE FIXED SATELLITE (E-S) FIXED FIXED EARTH EXPLORATION SAT. (Passive) SPACE RES. (Passive) SPACE RESEARCH (Passive) RADIO ASTRONOMY EARTH EXPLORATION SATELLITE (Passive) MOBILE FIXED MOBILE FIXED MOBILE FIXED FIXED SATELLITE (S-E) FIXED SATELLITE(S-E) FIXED SATELLITE (S-E) EARTH EXPL. SAT. (Passive) SPACE RES. (Passive) Radio- location Radio- location RADIO- LOCATION AMATEUR AMATEUR SATELLITE Amateur Amateur Satellite EARTH EXPLORATION SATELLITE (Passive) SPACE RES. (Passive) MOBILE MOBILE SATELLITE RADIO- NAVIGATION RADIO- NAVIGATION SATELLITE MOBILE MOBILE FIXED RADIO- ASTRONOMY FIXED SATELLITE (E-S) FIXED 3.0 3.025 3.155 3.230 3.4 3.5 4.0 4.063 4.438 4.65 4.7 4.75 4.85 4.995 5.003 5.005 5.060 5.45 MARITIME MOBILE AMATEUR AMATEUR SATELLITE FIXEDMobile MARITIME MOBILE STANDARD FREQUENCY & TIME SIG NAL (20,000 KHZ) Space Research AERONAUTICAL MOBILE (OR) AMATEUR SATELLITE AMATEUR MET. SAT. (S-E) MOB. SAT. (S-E) SPACE RES. (S-E) SPACE OPN. (S-E) MET. SAT. (S-E) Mob. Sat. (S-E) SPACE RES. (S-E) SPACE OPN. (S-E) MET. SAT. (S-E) MOB. SAT. (S-E) SPACE RES. (S-E) SPACE OPN. (S-E) MET. SAT. (S-E) Mob. Sat. (S-E) SPACE RES. (S-E) SPACE OPN. (S-E) MOBILE FIXED FIXED Land Mobile FIXED MOBILE LAND MOBILE LAND MOBILE MARITIME MOBILE MARITIME MOBILE MARITIM E MO BILE MARITIME MOBILE LAND MOBILE FIXED MOB ILE MOBILE SATELLITE (E-S) Rad ioloc ation Radiolocation LAND MOBILE AMATEUR MOBILE SATELLITE (E-S) RADIONAVIGATION SATELLITE MET. AIDS (Radiosonde) METEOROLOGICAL AIDS (RADIOSONDE) SPACE RESEARCH (S-S) FIXED MOBILE LAND MOBILE FIXED LAND MOBILE FIXED FIXED RADIO ASTRONOMY RADIO ASTRONOMY METEOROLOGICAL AIDS (RADIOSONDE) METEOROLOGICAL AIDS (Radiosonde) METEOROLOGICAL SATELLITE (s-E) Fixed FIXED MET. SAT. (s-E) FI XED FIXED AERONAUTICAL MOBILE SATELLITE (R) (space to Earth) AERONAUTICAL RADIONAVIGATION RADIONAV. SATELLITE (Space to Earth) AERONAUTICAL MOBILE SATELLITE (R) (space to Earth) Mobile Satellite (S- E) RADIO DET. SAT. (E-S) MO B IL ES AT (E - S) AERO. RADIONAVIGATION AERO. RADIONAV. AERO. RADIONAV. RADIO DET. SAT. (E-S) RADIO DET. SAT. (E-S) MOBILE SAT. (E-S) MOBILE SAT. (E-S) Mobile Sat. (S-E) RADIO ASTRONOMY RADIO ASTRONOMY MOBI LE SAT. (E- S) FIXED MOBILE FIXED FI XE D (LO S) MOBILE (LOS) SPACE RESEARCH (s-E)(s-s) SPACE OPERATION (s-E)(s-s) EARTH EXPLORATION SAT. (s-E)(s-s) Amateur de x i F ELIBOM RADIOLOCATION AMATEUR RADIO ASTRON. SPACE RESEARCH EARTH EXPL SAT FIXE D SAT. (S-E) FIXED MOBILE FIXED SATELLITE (S-E) FIXED MOBILE FIXED SATELLITE (E-S) FIXED SATELLITE (E-S) MOBILE FIXED SPACE RESEARCH (S-E) (Deep Space) AERONAUTICAL RADIONAVIGATION EARTH EXPL. SAT. (Passive) 300 325 335 405 415 435 495 505 510 525 535 1605 1615 1705 1800 1900 2000 2065 2107 2170 2173.5 2190.5 2194 2495 2501 2502 2505 2850 3000 RADIO- LOCATION BROADCASTING FIXED MOBILE AMATEUR RADIOLOCATION MOBILE FIXED MARITIME MOBILE MARITIME MOBILE (TELEPHONY) MARITIME MOBILE LAND MOBILE MOBILE FIXED 30.0 30.56 32.0 33.0 34.0 35.0 36.0 37.0 37.5 38.0 38.25 39.0 40.0 42.0 43.69 46.6 47.0 49.6 50.0 54.0 72.0 73.0 74.6 74.8 75.2 75.4 76.0 88.0 108.0 117.975 121.9375 123.0875 123.5875 128.8125 132.0125 136.0 137.0 137.025 137.175 137.825 138.0 144.0 146.0 148.0 149.9 150.05 150.8 152.855 154.0 156.2475 157.0375 157.1875 157.45 161.575 161.625 161.775 162.0125 173.2 173.4 174.0 216.0 220.0 222.0 225.0 235.0 300 ISM – 6.78 ± .015 MHz ISM – 13.560 ± .007 MHz ISM – 27.12 ± .163 MHz ISM – 40.68 ± .02 MHz ISM – 24.125 ± 0.125 GHz 30 GHz ISM – 245.0 ± 1GHz ISM – 122.5 ± .500 GHz ISM – 61.25 ± .250 GHz 300.0 322.0 328.6 335.4 399.9 400.05 400.15 401.0 402.0 403.0 406.0 406.1 410.0 420.0 450.0 454.0 455.0 456.0 460.0 462.5375 462.7375 467.5375 467.7375 470.0 512.0 608.0 614.0 698 746 764 776 794 806 821 824 849 851 866 869 894 896 901901 902 928 929 930 931 932 935 940 941 944 960 1215 1240 1300 1350 1390 1392 1395 2000 2020 2025 2110 2155 2160 2180 2200 2290 2300 2305 2310 2320 2345 2360 2385 2390 2400 2417 2450 2483.5 2500 2655 2690 2700 2900 3000 1400 1427 1429.5 1430 1432 1435 1525 1530 1535 1544 1545 1549.5 1558.5 1559 1610 1610.6 1613.8 1626.5 1660 1660.5 1668.4 1670 1675 1700 1710 1755 1850 MARITIME MOBILE SATELLITE (space to Earth) MOBILE SATELLITE (S-E) RADIOLOCATION RADIONAVIGATION SATELLITE (S-E) RADIOLOCATION Amateur Radiolocation AERONAUTICAL RADIONAVIGATION SPA CE RESEARCH ( Passive) EARTH EXPL SAT (Passive) RAD IO A STR ONOM Y MOBILE MOBI LE ** FIXE D-SAT ( E-S) FIX ED FIXED FIX ED** LAND MOBILE (TLM ) MOBILE SAT. (Space to Earth) MARITIME MOBILE SAT. (Space to Earth) Mobile (Aero. TLM) MOBILE SATELLITE (S-E) MOBILE SATELLITE (Space to Earth) AERONAUTICAL MOBILE SATELLITE (R) (space to Earth) 3.0 3.1 3.3 3.5 3.6 3.65 3.7 4.2 4.4 4.5 4.8 4.94 4.99 5.0 5.15 5.25 5.35 5.46 5.47 5.6 5.65 5.83 5.85 5.925 6.425 6.525 6.70 6.875 7.025 7.075 7.125 7.19 7.235 7.25 7.30 7.45 7.55 7.75 7.90 8.025 8.175 8.215 8.4 8.45 8.5 9.0 9.2 9.3 9.5 10.0 10.45 10.5 10.55 10.6 10.68 10.7 11.7 12.2 12.7 12.75 13.25 13.4 13.75 14.0 14.2 14.4 14.47 14.5 14.7145 15.1365 15.35 15.4 15.43 15.63 15.7 16.6 17.1 17.2 17.3 17.7 17.8 18.3 18.6 18.8 19.3 19.7 20.1 20.2 21.2 21.4 22.0 22.21 22.5 22.55 23.55 23.6 24.0 24.05 24.25 24.45 24.65 24.75 25.05 25.25 25.5 27.0 27.5 29.5 29.9 30.0 ISM – 2450.0 ± 50 MHz 30.0 31.0 31.3 31.8 32.0 32.3 33.0 33.4 36.0 37.0 37.6 38.0 38.6 39.5 40.0 40.5 41.0 42.5 43.5 45.5 46.9 47.0 47.2 48.2 50.2 50.4 51.4 52.6 54.25 55.78 56.9 57.0 58.2 59.0 59.3 64.0 65.0 66.0 71.0 74.0 75.5 76.0 77.0 77.5 78.0 81.0 84.0 86.0 92.0 95.0 100.0 102.0 105.0 116.0 119.98 120.02 126.0 134.0 142.0 144.0 149.0 150.0 151.0 164.0 168.0 170.0 174.5 176.5 182.0 185.0 190.0 200.0 202.0 217.0 231.0 235.0 238.0 241.0 248.0 250.0 252.0 265.0 275.0 300.0 ISM – 5.8 ± .075 GHz ISM – 915.0 ± 13 MHz INTER-SATELLITE RADIOLOCATION SATELLITE (E-S) AERONAUTICAL RADIONAV. PLEASE NOTE: THE SPACING ALLOTTED THE SERVICES IN THE SPEC- TRUM SEGMENTS SHOWN IS NOT PROPORTIONAL TO THE ACTUAL AMOUNT OF SPECTRUM OCCUPIED. AERONAUTICAL MOBILE AERONAUTICAL MOBILE SATELLITE AERONAUTICAL RADIONAVIGATION AMATEUR AMATEUR SATELLITE BROADCASTING BROADCASTING SATELLITE EARTH EXPLORATION SATELLITE FIXED FIXED SATELLITE INTER-SATELLITE LAND MOBILE LAND MOBILE SATELLITE MARITIME MOBILE MARITIME MOBILE SATELLITE MARITIME RADIONAVIGATION METEOROLOGICAL AIDS METEOROLOGICAL SATELLITE MOBILE MOBILE SATELLITE RADIO ASTRONOMY RADIODETERMINATION SATELLITE RADIOLOCATION RADIOLOCATION SATELLITE RADIONAVIGATION RADIONAVIGATION SATELLITE SPACE OPERATION SPACE RESEARCH STANDARD FREQUENCY AND TIME SIGNAL STANDARD FREQUENCY AND TIME SIGNAL SATELLITE RADIO ASTRONOMY FIXED MARITIME MOBILE FIXED MARITIME MOBILE Aeronautical Mobile STANDARD FREQ. AND TIME SIGNAL (60 kHz) FIXED Mobile* STAND. FREQ. & TIME SIG . MET. AIDS (Radiosonde) Space Opn. (S-E) MOBILE. SAT. (S-E) Fixed Standard Freq. and Time Signal Satellite (E-S) FI XE D STANDARD FREQ. AND TIME SIGNAL (20 kHz) Amateur MOB ILE FIXED SAT. (E-S) Space Research ALLOCATION USAGE DESIGNATION SERVICE EXAM PLE DESC RIPTION Prim ary FIXED Capital Le tters Sec on da ry Mobi le 1st Capital with low er ca se le tters U.S. DEPARTMENT OF COMMERCE National Telecommunications and Information Administration Office of Spectrum Management October 2003 MOBILE BROADCASTING TRAVE LERS INFOR MATION STATION S ( G) AT 161 0 k Hz 59-64 GHz IS DESIGNATED FOR UNLICENSED DEVICES Fixed AERONAUTICAL RADIONAVIGATION SPACE RESEARCH (Passive) * EXCEPT AERO MOBI LE ( R) ** EXCE PT AE RO M OBILE WAVELENGTH BAND DESIGNATIONS ACTIVITIES FREQUENCY 3 x 10 7 m 3 x 10 6 m 3 x 10 5 m 30,000 m 3,000 m 300 m 30 m 3 m 30 cm 3 cm 0.3 cm 0.0 3 cm 3 x 10 5 Å 3 x 10 4 Å 3 x 10 3 Å 3 x 10 2 Å 3 x 10Å 3Å 3 x 10 -1 Å 3 x 10 -2 Å 3 x 10 -3 Å 3 x 10 -4 Å 3 x 10 -5 Å 3 x 10 -6 Å 3 x 10 -7 Å 0 10 Hz 100 Hz 1 kHz 10 kHz 100 kHz 1 MHz 10 MHz 100 MHz 1 GHz 10 GHz 100 GHz 1 THz 10 13 Hz 10 14 Hz 10 15 Hz 10 16 Hz 10 17 Hz 10 18 Hz 10 19 Hz 10 20 Hz 10 21 Hz 10 22 Hz 10 23 Hz 10 24 Hz 10 25 Hz THE RADIO SPECTRUM MAGNIFIED ABOVE 3 kHz 300 GHz VERY LOW FREQUENCY (VLF) Audible Range bisiV retemilliM-buSradaR tsacdaorB MF tsacdaorB MA le Ultraviolet Ga yar-cimsoC yar-amm Infra-sonics Sonics Ultra-sonics Microwaves Infrared P L S X C Radar Bands LF MF HF VHF UHF SHF EHF INFRARED VISIBLE ULTRAVIOLET X-RAY GAMMA-RAY COSMIC-RAY X-ray ALLOCATIONS FREQUENCY BROADCASTING FIXED MOBILE* BROADCASTING FIXED BROADCASTING FIXED Mobile FIXED BROADCASTING BROADCASTING FIXED FIXED BROAD CASTING FIXED BROADCASTING FIXED BROADCASTING FIXED BROADCASTING FIXED BROADCASTING FIXED BROADCASTING FIXED FIXED FIXED FIXED FIXED FIXED LAND MOB ILE FIXED AERONAUTICAL MOBILE (R) AMATE UR SATEL LITE AMA TEUR MOBIL E SATE LLITE (E-S) F I X E D F ix e d M o b il e R ad i o - l oc a ti o n F I X E D M O B I L E LAND MO BILE MARITIM E MO BILE FIX ED LAND MOBILE FIXED LAND MOBILE RADIONAV-SATEL LITE FIXED MOB ILE FIXED LAND MOBILE MET. AIDS (Ra dio- son de) SPACE OPN. (S-E) Earth Expl Sat (E-S) Met-Satellite (E-S) MET-SAT. (E-S) EARTH EXPL SAT. (E-S) Earth Expl Sat (E-S) Met-Satellite (E-S) EARTH EXPL SAT. (E-S) MET-SAT. (E-S) LAND MOBILE LAND MOBILEFIXED LAND MOBILE FIXED FIXED FIXED LAND MOBILE LAND MOBILE FIXED LAND MOBILE LAND MOBILE LAND MOBILE LAND MOBILE MOBILEFIXED MOBILEFIXED BROADCASTMOBILE FIXED MOBILEFIXED FIXED LAND MOBILE LAND MOBILE FIXEDLAND MOBILE AERONAUTICAL MOBILE AERONAUTICAL MOBILE FIXED LAND MOBILE LAND MOBILE LAND MOBILE FIXED LAND MOBILE FIXED MOB ILE FIXED FIXED FIXED MOBILE FIXED FIXED FIXED BROADCAST LAND MOBILE LAND MOBILE FIX ED LAND MOBILE METEOROLOGICAL AIDS FX Space res. Radio Ast E-Expl Sat FIXED MOBILE** MOBIL E SATE LLITE (S-E) RADIODETERMINATION SAT. (S-E) Radiolocation MOBILE FIXED Am at eurRadiolocation AM ATEUR FIXED MOBILE B-SAT FX MOB Fixed Mobile Ra dio loc ati on RA DI OL OC AT IO N MOBI LE ** Fixed (TLM) LAND MOBILE FIXED (TLM) LAND MOBILE (TLM) FIXE D-SAT ( S-E) FIXED (TLM) MOBILE MOBILE SAT. (Space to Earth) Mob ile * * MOBILE** FIXED MOBILE MOBILE SATELLITE (E-S) SPAC E OP. (E-S)( s-s) EARTH EXPL. SAT. (E-S)(s-s) SPACE RES. (E-S)( s-s) FX. MOB. MOBILEFIXED Mobile R- LOC. BCST-SATELLITE Fixed Radio- location B-SAT R- LOC. FX MOB FixedMobile Ra dio loc ati on FIXEDMOBILE** Amateur RA DIO LOC ATIO N SPACE RES (S-E) MOBILEFIXED MOBILE SATELLITE (S-E) MARITIME MOBILE Mobile FIXED FIXED BROADCAST MOBILE FIXED MOBILE SATELLITE (E-S) FIXED FIXED MARI TIME MOB ILE FIXED FIXED MOBILE** FIXED MOBILE** FIXED SAT (S-E) AERO. RADIONAV. FIXED SATELLITE (E-S) Amateur- sat (s-e) Amateur MO BI LE FIX ED SAT( E-S) FI XE D FIXE D S ATELLIT E ( S-E)(E -S) FIXED FIXE D SAT ( E-S) MO BI LE Radio- location RADIO- LOCATION FIX ED SAT.(E-S ) Mo bil e** Fi xe d Mob ile FX SAT.(E -S) L M Sat(E-S) AER O RADI ONAV FIX ED SAT (E -S) AERONAUTICAL RADIONAVIGATION RADIOLOCATION Spac e Res.( act.) RADIOLOCATION Radiolocation Radioloc. RADIOLOC. Eart h Expl Sat Sp ace R es. Radiolocation BCS T SAT. FI XE D FIXE D SATE LLITE ( S-E) FIX ED S ATELLIT E (S -E) EARTH EX PL. SAT. FX SAT (S- E) SPACE RES. FIX ED S ATELLIT E (S -E) FIX ED S ATELLIT E (S -E) FIX ED S ATELLIT E (S -E) MOBILE SAT. (S-E) FX SAT (S- E) MOBIL E SATEL LITE (S-E) FX SAT (S- E) STD FREQ. & TIME MOBILE SAT (S-E) EARTH EX PL. SAT. MO BI LE FI XE D SPACE RES. FI XE D MO BI LE MO BI LE ** FI XE D EARTH EX PL. SAT. FI XE D MOBILE** RA D .A ST S PA CE RE S . FI XE D MO BI LE INTER-SATELLITE FI XE D RADI O ASTR ONOMY SPAC E RES . (P ass ive ) AMATEUR AMATEUR SATELLITE Radio- location Amateur RADIO- LOCATION Ear th Expl. Sa te ll i te (A ctiv e) FI XE D INTER-SATELLITERADIONAVIGATION RADIOLOCATION SATELLITE (E-S) INTER-SATELLITE FI XE D SAT EL LIT E (E -S) RADIONAVIGATION FI XE D SAT EL LIT E (E -S) FI XE D MOBILE SATELLITE (E-S)FIX ED S ATELLIT E (E -S) MO BI LE FI XE D Ear th Explora tion Sat ellit e (S- S) std freq & time e-e-sat (s- s) MO BI LE FI XE D e-e- sat MO BI LE SPACE RESEARCH (deep space) RADIONAVIGATION INTER - S AT SPACE RE S. FI XE D MO BI LE SPACE RES EARC H (spa ce-t o-Ear th) SP ACE RE S. FI XE D SAT. ( S-E) MO BI LE FI XE D FI XED -SAT ELL ITE MOBILE FI XE D FI XE D SAT EL LIT E MO BI LE SA T. FI XE D S AT MO BI LE SA T. EAR TH EX PL SAT ( E-S) Eart h Exp l. Sat (s - e) SPACE RES. (E-S) FX -S AT (S- E) FIXED MOBILE BR OA D- CA ST IN G B CS T SA T. RADIO ASTRONOMY FI XE D MO BI LE * * FI XE D SATEL LITE (E-S ) MO BI LE SATEL LITE (E-S ) FI XE D SATEL LITE (E-S ) MO BI LE RA DI ON AV. SAT EL LIT E FI XE D MO BI LE MOB. SAT(E-S) RA DIO NAV.S AT. MO BI LE SAT ( E-S). FI XE D MO BI LE F X SAT (E- S) MO BI LE FI XE D INTER - S AT EART H EXP L-SAT (P assiv e) SPACE RES. INTER - S AT SPACE RES. EA RT H- ES INTER - S AT EA RT H- ES SPACE RES. MO BI LE FI XE D EARTH EXPLORATION SAT. (Passive) S PA C E RES. MO BI LE FI XE D INT ER - SAT FI XE D MO BI LE INTER- SAT RADIO- LOC. MO BI LE FI XE D EARTH EXPLORATION SAT. (Passive) MOBILE FI XE D INTER- SATELLITE FI XE DMO BI LE* * MO BI LE ** INTER- SATELLITE MO BI LE INTER- SATELLITE RADIOLOC. Amateur Amateur Sat. Amateur RADIOLOC. AMATEUR SATAMATEURRADIOLOC. SPACE RESEARCH (Passive) EARTH EXPL SAT. (Passive) FI XE D MO BI LE INTER- SATELLITE SPACE RESEARCH (Passive) EARTH EXPL SAT. (Passive) Amatuer FI XE D M O- BI LE INTER- SAT. SPACE RES. E A R T H EXPL . SAT INTER- SATELLITE INTER-SAT. INTER-SAT. MOBILE FIXED FX-SAT (S - E) BCST - SAT. B- SAT. MOB** FX-SAT SPACE RESEARCH SPACE RES This cha rt is a graphic single-poin t-in-time portrayal of the Table o f Frequency Allocat ions used by the FCC and NTIA. As such, it doe s not complet ely reflect a ll aspects, i.e., footnotes and recent changes made to the Table of Frequency Allocations. Therefore, for complete in formation, users should consult the Table to dete rmine the curr ent status of U.S. allocations . Figure 5.1: The United States Spectrum Allocation. Wi-Fi operates in the circled bands 104 Chapter 5 www.newnespress.com Clearly, the array of allocations within the spectrum is bewildering. And network operators for licensed wireless technologies must be aware of the rules for at least the part of the spectrum that their technology works in, to avoid violating the terms of the license. But, thankfully, all of this is taken care of automatically when 802.11 technology is used. Wi-Fi operates in two separate stretches (or “bands”) of the radio spectrum, known in the United States as the Industrial, Scientific, and Medical (ISM) bands, and the Unlicensed National Information Infrastructure (U-NII) bands. These bands have a long history, and it is no coincidence that voice lead the way. Many people first became familiar with the concept of unlicensed radio transmissions when 900MHz cordless telephones were introduced. These phones require no licenses, but have a limited range and do only one thing—connect the call back to the one and only one base station. However, the power from using wireless to avoid having to snake cables throughout the house and allowing callers to walk from room to room revealed the real promise of wireless and mobility. For enterprises, the benefits of the freedom from using unlicensed spectrum are clear. Removing the regulatory hurdles from wireless brings the requirements for setting up wireless networks down to the same level as for wireline networks. Expanding the network, or changing how it is configured, requires no permission from outside authorities (ignoring the physical requirements such as building codes necessary to pull cables). There is no concern that a regulatory agency might reject a Wi-Fi network because of too many neighboring allocations. Enterprises gain complete control of their air, to deploy it how they see fit. Because being unlicensed gave the potential for every user to be her own network operator, wireless networking settled into the hands of the consumer, and that is where we will continue the story. 5.1.2 The Nearly Universal Presence Even though the focus of this book—and of so many people—is with enterprise and large- scale deployments, in explaining what makes Wi-Fi compelling, we must not lose track of the consumer, and how consumer demands have pushed the entire Wi-Fi industry forward, inevitably benefiting the enterprise. The major contribution the consumer space has given Wi-Fi is that is has driven people to demand wireless. Three historic events changed the landscape of mobility and connectivity: the Internet moved into the home; laptops replaced desktops and were being issued by corporate IT for usage everywhere; and darkly roasted coffee came onto the scene. Or rather, for the last one, people began to find reasons to want to work and live outside of the home and office. All three demanded a simpler solution than having to drag oversized telephone cables around with each user. And that gap was filled with Wi-Fi. Introduction to Wi-Fi 105 www.newnespress.com Wi-Fi is now in many places that mobile users are expected to show up in. In the home, it is difficult now to find a consumer-level gateway that does not include wireless. Just as television once was the centerpiece of the living room, but contention over control of the remote and the drop in prices lead televisions to spring up in nearly every room of the house, the Internet has migrated from being connected to one prized home computer in the living room to being spread throughout the house by Wi-Fi. In the enterprise, the advantages of unwiring the network edge has lead to IT organizations peppering the office with access points. And on the road, hotels, airports, cafes, and even sporting arenas have outfitted with Wi-Fi, to try to encourage their customers to get back with their online selves as often as possible, and maybe make each one be a little more “sticky” in the meanwhile. What this means for voice mobility is that the cycle of demand drives the technology to get ever better. Consumers’ demand and expectations “pull” advanced wireless into the home, just as enterprises “push” laptops onto their employees, encouraging them to be used outside the office, therefore increasing the number of hours employees think and do their work far beyond the amount of time each employee spends in the office. And with this cycle of demand also comes maturity of the underlying technology. Wi-Fi has gone through a number of iterations, getting faster, more powerful, and less prone to mistakes. Now, it is nearly impossible to find laptops without wireless built in. It is even an option on many desktop systems, not considered to be traditionally mobile, yet eager to be joined in on the wireless bandwagon to help company’s save on cabling costs. 5.1.3 Devices Wi-Fi was initially thought of as a data network only. Partially, this was because of an attempt to avoid the bad image that cordless phones also projected, as users were far too used to static and interference on cordless phones. But mostly, the original iterations of Wi-Fi occurred when Wi-Fi itself was struggling to find a place, and allowing users to check email or surf the Web while moving from room to room seemed to be enough of an application to motivate the fledgling industry. But when mobile data networking took off, and people became addicted to remote email over the cellular network, the seeds were sown for device vendors to want to integrate Wi-Fi into their mobile devices. And because those devices are primarily phones, the connection of mobility to voice over Wi-Fi was natural. Broadly, there are two categories of voice mobility devices that use Wi-Fi as a connection method. The first are Wi-Fi-only devices. These devices are often dedicated for a specific application in mind. For example, Vocera Communications makes a Wi-Fi-based communicator that is often used in hospitals to allow doctors and nurses to communicate with each other using voice recognition, rather than a keypad, to determine whom to call. 106 Chapter 5 www.newnespress.com This device looks and acts more like a Star Trek communicator than a phone, but is an excellent example of voice mobility within a campus. Polycom, through its SpectraLink division, Cisco, and Ascom all make handsets that look more like a traditional mobile phone. In all of these cases, single-mode networking—using just Wi-Fi, in these examples, as the only means of connectivity—makes sense for the environment and the application. The second type is made of mixed-mode, or integrated devices. These devices are mobile phones, made to be used with the cellular network as well as Wi-Fi. Nearly every mobile handset manufacturer is selling or is planning on selling such a device, including Research in Motion, Nokia, Samsung, and Apple with its iPhone. These devices can be made to place voice calls directly over the Wi-Fi network, rather than the cellular network, thus unlocking the entire fixed-mobile convergence (FMC) industry. In both cases, the push from Wi-Fi networks originally designed for data allows for voice to become a leading, if not the dominating, purpose for many networks, as the maturity and variety of Wi-Fi-enabled voice devices make voice mobility over Wi-Fi possible. 5.2 The Basics of Wi-Fi Wireline technologies are almost entirely focused on the notion of the cable. On one end lies the network, and on the other lies the client device. Starting with the original wireless telephone system, where everything—including identity—is determined merely by which port the cable connects to, the wireline technologies have only partially moved towards mobility and the concepts of link independence. However, Wi-Fi has no cables to begin with, and so something else is needed to define the relationship between a client and the network. Wi-Fi is built upon the notion of two types of wireless devices: the access point and the client. Both use the same types of radios, but take on different roles. 5.2.1 Access Points The access point (abbreviated as “AP”; see Figure 5.2) serves as the base station. The concept is common, from cordless phones to the large wireless carriers: the access point is what provides the “network,” and the clients connect to it to gain access. Each Wi-Fi radio, whether it be in the access point or the client, is designed to send its wireless signals across a limited range, far enough to be useful but not so far as to violate the limits set by the regulations and to grossly exceed the bounds of the building the network is deployed within. This range is in the order of 100 feet, though. To set apart which device connects to the network, the access point must take on a role as some sort of master. An access point often looks like a small brick, but with antennas and an Ethernet cable. The Ethernet cable provides the connection to the wired network, and, if power over Ethernet Introduction to Wi-Fi 107 www.newnespress.com Figure 5.2: A typical Access Point Ceiling Mounted Wall Mounted Mounted Above the Ceiling Figure 5.3: Typical Access Point mounting locations (PoE) is in use, the access point receives its power over the same cable. Access points are normally independent physical devices. Commonly, they are placed along walls, or above or below a false ceiling, to provide the maximal amount of wireless coverage with the least amount of physical impediments to the signals (see Figure 5.3). How exactly those locations are determined will be addressed later in this chapter. Access points make their networks known by sending frequent wireless transmissions, known as beacons. These beacons describe to the client devices what capabilities the access point has, and most importantly, what network the access point is providing access to. The way the network is designated is by an arbitrary text string provided by the administrator, known as a service set identifier (SSID). This text string is sent in the beacons, and other transmissions, to the clients, which then provide a list of SSIDs seen to the user. Thus, when the user brings up a list of the networks that his or her laptop sees and can connect to, the list contains the SSIDs of the access points. Because the SSID is the only way users can select which network they wants to connect to, we need to look into it a bit deeper. There are very few technical restrictions on the SSIDs 108 Chapter 5 www.newnespress.com except for the length, which must be less than 32 characters. However, the SSID needs to be meaningful to the user, or else he or she will not connect to it. Because SSIDs are supposed to name the network that the user is connecting to, rather than the individual access point, multiple access points can and do share the same SSID. That being said, there is nothing stopping someone else from giving an access point the SSID that belongs to your network. There is no security in the SSID itself. Eavesdroppers can trivially discover what the SSID is that your network is using (even if you use a feature known as SSID hiding or SSID broadcast suppression) and use it to either gain entry into your network or spoof your network and try to fraudulently get your clients to connect to them instead. In fact, there is nothing that prevents SSIDs from being used for nearly any purpose at all. Most of what applies to SSIDs are in the form of best practices, of which the important ones are: • TheSSIDshouldbemeaningfultotheuser:“employees”and“guest”aregoodexamples of meaningful names. They may be based on the role of the user, the device the user has (such as “voice” for phones), or any other words that help the user find the network. • Whentheinstallationsharestheairwithneighboringnetworksfromotherorganizations, the SSID should also include text to highlight to the user what the right network is; “xyz-employees” is an example of an SSID for an organization named XYZ. • TheSSIDshouldbeabletobeeasilytypedbytheuser.Althoughmostdevicesshow SSIDs in a list from what already are being broadcasted, allowing the user to select the SSID with minimal effort, there are many occasions on which when the user may need to type the SSID. This is especially true for mobile devices, with small keyboards or limited keys. • Again,donotrelyonobscurityoftheSSIDtorestrictaccesstoyournetwork.Usereal security mechanisms, as described later, instead. 5.2.2 Clients A client is the typical end-user device. Unlike access points, which are strategically placed for coverage, clients are almost always mobile (or potentially so). Wi-Fi clients can be general networking interface devices, such as those in laptops, or can be part of a purpose-built mobile voice handset. Either way, these clients appear to the network as endpoints, just as Ethernet devices do. From the user’s perspective, however, Wi-Fi clients add an extra complication. Unlike with wireline connections, where the user is assigned a port or cable and has the expectation that everything will work once the cable is plugged in and the process has settled down (which, for administrators, generally means that Dynamic Host Configuration Protocol (DHCP) automatic IP address discovery has completed), wireless connections have no one cable to Introduction to Wi-Fi 109 www.newnespress.com solve all problems. The user must be involved in the connection process, even when the reason for connection or disconnection is not readily apparent. As mentioned previously, the user must learn about SSIDs. When a wireless interface is enabled, the user is normally interrupted with a list of the available networks to connect to. Knowing the right answer to this question requires an unfortunate amount of sophistication from the user, not because the user does not understand the technology, but because they usually do understand the power of mobility, and have learned to strategically hunt out wireless networks for casual email access. This is clearly evidenced by the pervasive nature of the “Free Public WiFi” ad hoc (Independent Basic Service Set, or IBSS) SSID that tends to be on so many laptops. Ultimately, the user is responsible for knowing what the appropriate network is to connect to at any given location. Most devices do remember previous connections—including authentication credentials, in many cases—and can make the connection appear to be automatic. However, because of that caching, installations that run multiple SSIDs are often forced to deal with users not knowing exactly which network they are connected to. Once the connection is established, the interface comes up much as a plugged-in Ethernet link does. Any automatic services, such as DHCP or Universal Plug and Play (UPnP), that run on interface startup will get kicked off, and the users will be able to communicate as if they had plugged directly into the network. The last wrinkle comes, however, with mobility. Once the user leaves the coverage range of the one access point that it is on, the client will perform its list gathering activity (scanning) again. If it can find an SSID that it already has in its list—especially if the SSID is the same as the one the client was already associated to—the client will try to hand over to the new access point without user intervention. However, if the handoff does not succeed, or there are no more known networks in range, the client will disconnect and either warn the user with a popup or just break the connection without warning. This can come as quite a shock to the user, and can lend negative impressions about the network. 5.2.3 The IEEE 802.11 Protocol Now that we have the basic roles established, let’s look at the protocol itself. 5.2.3.1 Frame Formats Because it belongs to the IEEE 802 family of standards, 802.11 integrates tightly into existing Ethernet networks. Wi-Fi transmissions, like their wired Ethernet brethren, are contained in what are known as frames. In the IEEE 802 context (including the 802.3 Ethernet series as well as 802.11), a frame is one continuous transmission of data. For 802.11, as with Ethernet, these frames usually carry a payload of 1500 bytes or less. This payload can contain one IP packet. 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